Generally, in order to efficiently convert velocity energy of a fluid into pressure energy, a centrifugal pump is required to have a mechanism for decreasing a velocity of the fluid discharged from an impeller so as to recover a pressure head. Further, a multistage centrifugal pump having a plurality of impellers disposed in series is required to have a mechanism for leading a fluid from a certain-stage impeller to a next-stage impeller.
Thus, in a multistage centrifugal pump, a guide vane is widely used as a mechanism for decreasing a velocity of a fluid and leading the fluid to a next-stage impeller. This guide vane comprises diffuser passages for decreasing a velocity of a fluid discharged from an impeller, and return passages for leading the fluid, which has passed through the diffuser passages, to the next-stage impeller, as disclosed in the Japanese laid-open utility model publication No. 6-40958.
Since the above-mentioned diffuser passages and the return passages have a complicated shape, it has been customary to form the guide vane by using resin or by casting. The resin guide vane and the cast guide vane can have smooth passages therein for leading the fluid to a suction port of the next-stage impeller, and hence an excellent pump performance can be obtained.
However, the resin guide vane may be corroded depending on the characteristics of the fluid. Therefore, the types of fluids which the pump can handle are limited. Further, if the pump is used for delivering waste water, the resin guide vane is worn by suspended substances such as sands contained in the waste water. On the other hand, in a case of using the cast guide vane, the corrosion and the wear can be prevented from occurring. However, the cast guide vane causes a manufacturing cost to increase.
In order to solve such problems, there has been proposed a centrifugal pump having a diffuser section for decreasing a velocity of a fluid discharged from a rotating impeller, a plurality of return vanes for leading the fluid, which has passed through the diffuser section, toward a discharge side, and a main plate to which the diffuser section and the return vanes are fixed, all of which are manufactured from a sheet metal.